Tetsu-to-Hagané
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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 88 (2002), No. 5

  • Current Researches on the Formation of Solidification Microstructure in Alloys

    pp. 229-235

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    Researches on the theories of microstructure formation in the solidification of alloys are reviewed. Firstly, critical solidification rates from stable to unstable and from unstable to absolutely stable interface according to the perturbation theory are quantitatively expressed. It shows that the microstructure changes from planar interface to cell structure and changes again planar interface. Furthermore, the formation of each microstructure such as planar interface, cell, dendrite and band structure is explained by the phase selection theory. Next, equations on primary dendrite arm spacing, which have been suggested till now, show almost similar form. Finally, transition from columnar dendrite to equiaxed dendrite regions in an ingot is expressed as a function of temperature gradient, which is educed from the interaction of supercooling and nucleation in front of the columnar dendrite.
  • Morphology Control of Zinc Deposits of Electrogalvanized Steel Sheets

    pp. 236-242

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    The visual appearance, press formability and the corrosion resistance of electrogalvanized steel sheets change depending on the morphology of deposited Zn. In this paper, the effects of plating factors such as the surface condition of steel substrate, electrolysis factors and the bath condition on the morphology of deposited Zn were systematically discussed both from a crystallographic viewpoint of the epitaxy between Zn/steel and from an electrochemical viewpoint of the overpotential for Zn deposition. The crystal orientation index of Zn basal plane and the crystal size of Zn are decreased with increasing the overpotential for Zn deposition. They are also decreased with decreasing the epitaxy between Zn/steel even when the overpotential is kept constant. When the overpotential for Zn deposition is increased, the surface roughness of deposited Zn becomes larger because of an increase in the inclination of Zn basal plane to steel substrate. When the epitaxy between Zn/steel is decreased without changing the overpotential, the surface roughness becomes smaller because of the decrease in crystal size of Zn although the inclination of Zn basal plane is somewhat increased.
  • Effects of Pressure and Inert Gas Top Blowing on the Rate of Molten Steel Hydrogen Desorption

    pp. 243-248

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    The rate of hydrogen desorption from liquid steel bath was measured to investigate the effect of pressure. Experiments of hydrogen desorption from molten steel were conducted using 30kg and 100kg scale induction furnace. Hydrogen desorption reaction was basically controlled by gas phase mass transfer over 1.4 kPa in this experimental condition. The effect of argon gas top blowing on to the free surface of molten steeel bath on the gas phase mass transfer coefficient was evaluated. Following dimensionless equation was obtained, assuming 0.33 power of Sc.
    Sh(H/dn)=0.002Re0.88Sc0.33(H/dc)1.39
    where H is the distance between free surface of melt bath and nozzle, dn, is inner diameter of nozzle, dc is inner diameter of crucible, Sh= kGdn/D Ar-X, Re=ugdn/ vg, Sc = Vg/DAr-X, kG is mass transfer coefficient of gas phase, DAr-X is diffusion coefficient of diffusing gas, ug is gas velocity, vg is kinetic viscosity of gas.
  • Flow Characteristics of Liquid and Fine Particles in the Lower Part of Blast Furnace

    pp. 249-255

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    Cold model experiments on the fine particles movement, fine particles accumulation and liquid flow in the lower part of blast furnace at the high rate injection of pulverized coal were carried out. From the experimental results of the cold model on the liquid flow, it was found that the accumulation of fine particles on the surface of deadman caused liquid flow channeling along the surface of deadman. Two-dimensional cold model on the two phase flow of gas and solid indicated that shell layer formation at the depth of raceway was conducive to accumulation of fine particles on the surface of deadman. By the one-dimensional cold model experiment of the moving bed on the two phase flow of gas and solid, it became clear that fine particles hold-up in packed bed decreased as the increase in bed descending velocity as well as decrease in fine particles feed rate or increase in superficial gas velocity.
    On the basis of above results, control methods of lower part permeability at higher injection rate of pulverized coal were discussed. Consequently, it was estimated that decrease in coke degradation in raceway and removal of shell layer were effective to ensure the gas flow through deadman.
  • Behaviour of Nozzle Clogging in Deoxidation of Molten Iron with Injecting Magnesium Vapour

    pp. 256-263

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    Deoxidation experiments of molten iron with injecting magnesium vapour produced in-situ by aluminothermic reduction of magnesia were carried out. MgO-Al pellets were charged in the immersion tube, which was put into the molten iron. Several injection holes or thin injection tubes were installed at the bottom and the side of the immersion tube, which was called as the injection nozzle. The injection nozzle was often clogged during the deoxidation experiments.
    Effects of material, shape and number of gas exits of the nozzle, pellet composition and initial oxygen concentration on the nozzle clogging were investigated. The nozzle clogging took place earlier with increasing initial oxygen concentration in the molten iron. The Al2O3, MgO and mullite injection tubes were clogged more easily than the ZrO2 injection tube. The main reason of the nozzle clogging was deposition of the deoxidation product on the inner wall of injection holes and tubes. It was also found that the aluminum suboxide gas was formed during the aluminothermic reduction of MgO. The suboxide gas formation can be suppressed by adding excess MgO to the MgO-Al pellet of molar ratio of 3 : 2.
    Countermeasures to prevent the nozzle clogging have been proposed in this study. Decreasing the partial pressure of magnesium vapour in the injection gas, increasing the gas velocity through the nozzle and suppression of the aluminum suboxide gas formation are effective for preventing the nozzle clogging. Countermeasures can also be achieved by dividing pellet charging to the nozzle into several portions.
  • Suppression of Mold Powder Entrapment Using Immersion Nozzle of Elliptic Cross Section

    pp. 264-269

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    Mold powder descends along the surface of the immersion nozzle due to pressure difference around the nozzle when an uneven molten steel flow suddenly crosses the nozzle. If the descending mold powder arrives at the port of the immersion nozzle, it is torn off to many small particles by the discharging molten steel and carried deep into the mold. This type of mold powder entrapment can be suppressed by decreasing the pressure difference around the nozzle. In this study, an immersion nozzle with elliptic cross section was chosen, and cold model experiments were carried out using salt water and silicone oil as the working fluids. The descending distance of the silicone oil simulating mold powder was smaller along the elliptic cylinder than along a circular cylinder. As the ratio of the minor axis to the major axis of the elliptic cylinder became smaller, the descent of the silicone oil was more effectively suppressed. The effect of the wettability of the elliptic cylinder on the descent of the silicone oil was also investigated. Poor wettability promoted the descent of silicone oil significantly.
  • Simultaneous Determination of Arsenic and Antimony in Steels and Pure Coppers by High Power Nitrogen Microwave Induced Plasma Atomic Emission Spectrometry Coupled with Hydride Generation Technique

    pp. 270-276

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    The feasibility of an annular-shaped high power nitrogen microwave induced plasma atomic emission spectrometry (N2-MIP-AES) has been studied for the simultaneous determination of arsenic and antimony in combination with the hydride generation method. Under the optimized experimental conditions, the best attainable detection limits at As I 228.812 and Sb I 231.147 nm lines were 4.13 and 4.50 ng/ml for arsenic and antimony, respectively, with a linear dynamic range of 10 to 10, 000 ng/ml in concentrations. The presence of several diverse elements was found to cause more or less a depressing interference by the proposed technique. Of the several pre-reductants examined, thiourea was found to be the most preferable to reduce arsenic and antimony from their pentavalent state to trivalent one prior to hydride generation. Therefore, thiourea was utilized as a pre-reductant for the determination of total arsenic and antimony concentration, i.e., As(III)+As(V) and Sb(III) + Sb(V). When arsenic and antimony in steels were determined simultaneously, a large amount of Fe(III) in the solution caused a severe depressing interference, while the presence of Fe(II) showed little or no significant interference. Of the several interference-releasing agents examined, both thiourea and L-ascorbic acid were found to be the most preferable to reduce Fe(III) to Fe(II). When arsenic and antimony in high purity coppers were determined simultaneously, copper of principal constituent interfered to a great extent. Thiourea was found to reduce the depressing interference from copper, so that any separation procedure of copper matrix was not necessary. The proposed method using thiourea not only as a pre-reductant but also as an interference-releasing agent was applied to the simultaneous determination of low concentrations of arsenic and antimony in carbon steels and high pure coppers. The results obtained by this method were in good agreement with the certified values.
  • Effect of S Segregation and BN Precipitation to Creep Cavities on Creep Rupture Properties

    pp. 277-283

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    Segregation of S and precipitation of BN on creep cavity surfaces during creep and their effects on creep rupture properties are reported. Auger electron spectroscopy of creep cavitated specimens provided direct evidence of impurity segregation to creep cavity surfaces. Residual S segregated most strongly, and was observed on creep cavity surfaces in crept specimens of a coventional type 304 stainless steel. The segregated S lowers the surface energy of creep cavities, and promotes nucleation of creep cavities. It was thought that the extensive segregation of S brings on extensive creep cavitation and premature failure with low ductility. In the modified 304 stainless steel added with B, N, Ce and Ti, the segregation of S and the creep cavitation were greatly inhibited. Addition of Ce and Ti lowered the amount of S available to segregate to creep cavity surfaces, thereby replacing the segregated S by precipitation of BN. BN is very stable at high temperatures and expected to reduce the surface diffusion rate considerably due to precipitating on creep cavity surfaces. It was suggested that this precipitation of BN on creep cavity surfaces is closely related to the suppression of creep cavitation and high rupture ductility of the modified steel.

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